A technique of mixing fibers in rubber to improve the hardness and the modulus of the rubber has already been known. Fibers having a large diameter may readily disperse in rubber but may lower the physical properties such as fatigue resistance and the like of rubber; but on the contrary, fibers having a small diameter could improve the fatigue resistance of rubber but may aggregate and may become entangled together to worsen the dispersibility thereof in rubber.
PTL 1 proposes a method of satisfying both dispersibility and fatigue resistance, which comprises dispersing blend fibers having a sea/island-structured cross-section in rubber and fibrillating the fibers by the shearing force given thereto during mixing to thereby increase the contact area of the resultant fibers with rubber.
PTL 2 discloses a fact that when a bacterial cellulose having a small diameter of 0.1 μm along with starch serving as a reinforcing agent is mixed in a dienic rubber for the purpose of improving the abrasion resistance of rubber, then the abrasion resistance index of the resultant mixture improves as compared with that in a case where starch alone is incorporated.